What are you doing when you aren't doing anything at all? If you said "nothing," then you have just passed a test in logic and flunked a test in neuroscience. When people perform mental tasks--adding numbers, comparing shapes, identifying faces--different areas of their brains become active, and brain scans show these active areas as brightly colored squares on an otherwise dull gray background. But researchers have recently discovered that when these areas of our brains light up, other areas go dark. This dark network (which comprises regions in the frontal, parietal and medial temporal lobes) is off when we seem to be on, and on when we seem to be off. If you climbed into an MRI machine and lay there quietly, waiting for instructions from a technician, the dark network would be as active as a beehive. But the moment your instructions arrived and your task began, the bees would freeze and the network would fall silent. When we appear to be doing nothing, we are clearly doing something. But what?

The answer, it seems, is time travel.

The human body moves forward in time at the rate of one second per second whether we like it or not. But the human mind can move through time in any direction and at any speed it chooses. Our ability to close our eyes and imagine the pleasures of Super Bowl Sunday or remember the excesses of New Year's Eve is a fairly recent evolutionary development, and our talent for doing this is unparalleled in the animal kingdom. We are a race of time travelers, unfettered by chronology and capable of visiting the future or revisiting the past whenever we wish. If our neural time machines are damaged by illness, age or accident, we may become trapped in the present. Alzheimer's disease, for instance, specifically attacks the dark network, stranding many of its victims in an endless now, unable to remember their yesterdays or envision their tomorrows.

Why did evolution design our brains to go wandering in time? Perhaps it's because an experience is a terrible thing to waste. Moving around in the world exposes organisms to danger, so as a rule they should have as few experiences as possible and learn as much from each as they can. Although some of life's lessons are learned in the moment ("Don't touch a hot stove"), others become apparent only after the fact ("Now I see why she was upset. I should have said something about her new dress"). Time travel allows us to pay for an experience once and then have it again and again at no additional charge, learning new lessons with each repetition. When we are busy having experiences--herding children, signing checks, battling traffic--the dark network is silent, but as soon as those experiences are over, the network is awakened, and we begin moving across the landscape of our history to see what we can learn--for free.